Fluoride containing chemistries have been used for many years with prime silicon wafers in the semiconductor industry. Patents that teach methods for cleaning prime wafers with low pH solutions include U.S. Pat. Nos. 5,560,857; 5,645,737; 5,181,985; 5,603,849; and 5,705,089.
After the Front End of Line (FEOL) cleaning process the wafer proceeds to the typical Back End of Line (BEOL) manufacturing process for a semiconductor devices, in which the devices might be dynamic random access memories (DRAMs), static random access memories (SRAMs), logic, electrically programmable read only memories (EPROMs), complementary metal on silicon (CMOS), and the like. Etching fabrication technology using chemical reactions (liquid or plasma) has been used as a method of forming a wiring structure on such semiconductor substrates. Normally, dilute hydrofluoric acid is also used as the last process step in the sequence called “RCA rinses,” to clean the substrate contaminated from previous process steps with metal, anions and/or organic residues. Using fluoride chemistries (usually HF) as a final RCA cleaning step will cause the silicon wafer surface to be in a hydrophobic state (the surface is covered with Si-H groups) which will repel water. During this step a certain proportion of the oxide surface is dissolved (removed). Unless the conditions are carefully monitored (time, temperature, solution composition) the substrates can be damaged.
A photoresist film is deposited on the wafer to form a mask, then a substrate design is imaged on the film layer, baked, and the undeveloped image is removed with a developer. The remaining image is then transferred to the underlying material through etching (either a dielectric or metal) with reactive etching gases promoted with plasma energy. The etchant gases selectively attack the unprotected area of the substrate. Liquid or wet etching chemistries have been used extensively over the years to etch metals, oxides and dielectrics. These chemistries provide aggressive isotropic etching (etching equally in all directions). Increasingly, plasma etching, reactive ion etching or ion milling are used, and such etching processes produce undesirable residues from the interaction of the plasma gases, reacted species and the photoresist. The composition of the resiudue is influenced by the type of etching, the resist, the underlying substrates, and the process conditions utilized.
If etching residue is not removed from the substrate, the residue can interfere with subsequent processes involving the substrate. The effect of poor cleaning results in low device yield, low device reliability, and low device performance.
To fulfill the demand for faster processing speed from semiconductor, the conventional Al or Al alloy used as the interconnection material has been replaced with Cu or Cu alloy, typically using a known damascene process. A barrier film, which may be silicon nitride, and a Low-k film are successively formed on the substrate, and a resist mask is then formed. The common low-K dielectrics include CORAL, tetraethylorthosilicate (TEOS), SiOC, porous MSQ, SiON, and boron phosphosilicate glass (BPSG). New low-k films having a lower dielectic constant than the p-TEOS film include a film formed of inorganic material such as porous silica or the like, a film formed organic material such as polyimide, polyarylene or the like, and a film formed of a mixture of the above-mentioned inorganic and organic materials. Next, the exposed low-k film is dry etched to expose the barrier film, so that a via hole is formed. At this time, reactive products of the gas used for the dry etching and the Low-k film and the resist film accumulate in the via hole as resist residue. Then, the resist film is removed by plasma ashing, leaving a modified film on the surface of Low-k film according to the reaction of the resist to heat and plasma during ashing. Then the resist residue is removed by processing with a fluoride-based cleaning composition. To ensure the removal of the resist residue, usually cleaning compositions have hydrofluoric acid that aggressively attacks the via sidewall of the dielectric and therefore changes the dimensions of the device, as taught by Ireland, P., Thin Solid Films, 304, pp. 1-12 (1997), and possibly the dielectric constant likely to evenly etch the insulating film has been used, and the via holes are enlarged. D. It is not uncommon for the HF to also attack the dielectric material. Such attack is not desirable (see Lee, C. and Lee, S., Solid State Electronics, 4, pp. 92 1-923 (1997)). Subsequently, a resist film patterned for trench formation is formed on the Low-k film, and, using the resist mask, the Low-k film is dry etched down to its intermediate position to form a trench. Resist residue that is the reactive product of the gas used for the dry etching and the Low-k film accumulates in the via hole and trench. The resist film is removed by plasma ashing, and resist residue is removed by processing with a conventional fluorine type compound-based cleaning composition. The conventional cleaning composition removes the resist residue and also etches the surface of the Low-k film, so that the internal diameter of via hole is further enlarged and the width of trench increases. Then, the barrier film, e.g., silicon nitride, is removed by dry etching to expose buried copper interconnections. Then, the surface of the copper interconnection is cleaned with a cleaning composition. In the conventional fluoride-based cleaning compositions, a copper corrosion inhibitor such as benzotriazole (BTA) has been added to prevent corrosion of the copper interconnection. With such a cleaning composition, however, there is a problem that the copper corrosion prevention interferes with attempts to improve the resist residue removing action. Finally, copper is filled in the via hole and trench by plating or the like.
The requirement for cleaning solutions that remove all types of residue generated as a result of plasma etching of various types of dielectrics and metals, such as aluminum, aluminum/silicon/copper, titanium, titanium nitride, titanium/tungsten, tungsten, silicon oxide, polysilicon crystal, etc., while not corroding or chemically altering the underlying dielectrics and often metals, presents a need for more effective chemistry in the processing area.
In addition, stripping compositions used for removing photoresist coatings and cleaning composition for removing post-etch residue have for the most part been highly flammable, generally hazardous to both humans and the environment, and comprise reactive solvent mixtures exhibiting an undesirable degree of toxicity. Moreover, these compositions are not only toxic, but their disposal is costly since they might have to be disposed of as a hazardous waste. In addition, these compositions generally have severely limited bath life and, for the most part, are not recyclable or reusable.
Numerous formulations have been described that purport to reduce oxide loss while maintaining cleaning ability. These compositions combine one or more of HF, ammonium fluoride, and or ammonium bifluoride as the fluoride source, in addition to water, organic solvents, and various additives. The mechanisms used to control corrosion and attack of dielectrics include reducing water content, reducing fluoride content (as HF) to less than 0.2%, usually less than 0.1%, and control the pH to be near neutral, e.g., between 4 and 10, usually between 5 and 9.
Water typically increases HF corrosion-: silica has an etch rate of 21 Å/min (@ 25° C.) in HF/water, but in HF/isobutanol the etch rate is only 2.1 Å/min, and in HF/acetone the etch rate is only 0.12 Å/min, as reported at NSF/SRC Eng. Res. Center, Environmentally Benign Semiconductor Manufacturing, Aug. 5-7, 1998, Stanford University. Several low-k materials can be altered by water. Polar organic solvents, typically amides, ethers, or mixtures thereof, can partially or completely reduce water. These solvents may be necessary for some substrates, but they are expensive and pose disposal problems.
Dilute compositions can lose effectiveness after repeated use, for example in baths. Recent information also indicates that the dilute HF solutions can be ineffective for cleaning the newer CFx etch residues, as taught by K. Ueno et al., “Cleaning of CHF3 Plasma-Etched SiO2/SiN/Cu Via Structures with Dilute Hydrofluoric Acid Solutions,” J. Electrochem. Soc., vol. 144, (7) 1997. Contact holes opened on to the TiSi2 have also been difficult to clean with dilute HF solutions since there appears to be an attack of the underlying TiSi2 layer. There may also be difficulty with mass transport of the chemicals in the narrow hydrophilic contact holes, as taught by Baklanov, M. R. et al., Proc. Electrochem. Soc., 1998, 97-35, pp. 602-609.
Accordingly, there exists a need to develop improved cleaning compositions to efficiently clean a variety of deposits from a wide variety of substrates. Particularly in the field of integrated circuit fabrication, it should be recognized that the demands for improved cleaning performance with avoidance of attack on the substrates being cleaned are constantly increasing. This means that compositions that were suitable for cleaning less sophisticated integrated circuit substrates may not be able to produce satisfactory results with substrates containing more advanced integrated circuits in the process of fabrication. The cleaning compositions should also be economical, environmental friendly and easy to use.